The invention relates generally to programmable implantable biomedical devices.
Programming in the context of this application means noninvasively transferring parameter value data from an external device called the programmer to an internal device implanted in the patient's body. A number of systems have been successfully employed in commercially available cardiac pacers, including magnetic programming and radio frequency (RF) programming. One of the problems addressed by the present invention is expanding the number and range of parameters programmable by pulse counting
techniques such as have been used in the past in Cordis Omnicor.RTM. cardiac pacers.
Cardiac pacers are life supporting, therapeutic medical devices. They are surgically implanted and remain in a living person's body for years. The vital considerations in cardiac pacing technology tend to dictate a conservative approach. Thus, while pulse counting techniques are less efficient at transmitting data than more sophisticated pulse coding techniques, they have proven extremely reliable over many years of service.
The typical implanted cardiac pacer operates by supplying missing stimulation pulses on a pacing lead attached to the ventricle. The so called R-wave can be sensed by the same lead. An additional lead contacts the atrium to sense P-waves, if desired. In programmable pacers, the fixed rate at which the pulse generator will produce pulses may be selected from among a variety of optional rates, for example, from forty to one hundred beats per minute. It is desirable to have as many rates available as is practical for two reasons. First, it enhances the physician's ability to match the pacer to the patient so as to avoid angina and to coordinate the fixed rate with the patient's normal sinus rhythm. Secondly, the higher the rate, the shorter the life time of the pacer battery. By the same token, it is desirable to have a number of different pulse intensities available for selection. Pulse intensity is programmed either by adjusting pulse width or pulse current. It is generally desirable to minimize the pulse intensity to minimize tissue damage which changes the contact resistance and to conserve power.
The stimulation output of the pulse generator is applied to the heart via two electrodes, namely, a cathode and anode. Typically the cathode for ventricular pacing is located at the top of an elongated insulated pacing lead which extends pervenously into the right ventricle of the heart. The electrical return path to the anode can be achieved in two different ways. The case of the pulse generator can be used as the anode. In this system since only one electrode is located at the end of the pacer lead, the lead is called "unipolar". In the other system, the anode as well as the cathode are located at the end of the pacer lead which is referred to as a "bipolar" lead in this case.
In the past, cardiac pacers and pacing leads have been available in either unipolar or bipolar configuration. The anode location was irreversibly selected at implantation. If a unipolar lead was implanted, and it was determined that capture could be more easily obtained with a bipolar lead or if there were muscular twitching in the vicinity of the pulse generator, the patient had to undergo a new operation to replace both the lead and the pulse generator.